Thanks for writing up the anatomy of a marine surge. Most everything is made clear and easy to understand. However, this one behavior always seems counter-intuitive to me:

"A heat wave occurs as an upper level high pressure at 18K feet comes over California. This mass of descending air compresses and heats up the Valley. So the Central Valley low pressure enlarges and expands to the coast or beyond."

The model I had in my head is that when the Central Valley heats up at ground level, the hot air rises, creating low pressure (like creating a vacuum from air leaving and rising up from the heat).

But a high pressure mass of descending air at 18K feet, compressing, and heating up the Valley - this sort of goes against the "rising hot air creating a vacuum" model. It seems like a descending high pressure mass would cause high pressure, not low pressure. Just seems counter-intutive to me.

What makes the low pressure is not air just air rising creating a partial vacuum. The Central Valley is not a closed container so as it heats up the air expands in all directions not just up. But with the crest of sierra to the east most of that expansion is west towards the coast. And as the air expands there are fewer molecules per unit of volume hence the low pressure. All of this is happening at the surface on a typical summer day and the low pressure sucks the NW ocean wind so it curves into the Bay Area.

Now in a heat wave an upper ridge of high pressure at around 18,000 feet moves over California. Picture this as a thickening of the atmosphere creating high pressure aloft With low pressure at the surface in the valley the air aloft moves downward. And as this air descends towards the surface it compresses creating more heat (think of filling a scuba tank) below the upper high especially in the Central Valley. This in turn causes the Central Valley air to expand even more so hot low pressure air comes over the bay. In a strong heat wave this low pressure bulges beyond the Golden Gate pushing the North Pacific High's surface NW winds away from shore.

I find written explanations of weather patterns (and geology) most difficult to follow / envision (some of this reminds of me of reading John McPhee). The maps and especially the animations make it much more clear. & so thanks for that! Question: with all the fairly easy computer animation at our disposal, has anyone done a nice tutorial on all this? Still having a hard time with "troughs".

Mike... I appreciate the write-ups as well. Is it just me or are the forecasts clearly much better this year.

Hi Chang10,

We have had very simple patterns so far this year so the forecasts have been very accurate at most sites. Once we get into a mostly thermal summer pattern the forecast accuracy will go down.

jchang10 wrote:

my 2c worth is that the NPH is "displacing" the central valley low, not creating it. In this case, it displaces it by squashing it like a bubble or balloon so we get a low off the coast.
I have wondered how we can have these blistering NW winds moving north to south and into the bay, but at the same time clearly see this marine layer move from LA south to north at the same time.

I think you are confusing the NPH and the upper level high at 18K feet. The NPH creates the NW surface winds. The upper level high pressure is almost 2 miles overhead and it created the minor heat wave over the weekend.

Look at the animation above. Saturday and Sunday the North Pacific High's surface NW winds at the coast near the Bay. Then as the surface low pressure bulges out west of the Golden Gate you see the North Pacific Highs winds moving far from shore. It took longer than expected for the NW wind to retreat which was why the marine surge did not arrive until almost Sunday evening.

The low pressure west of the Golden Gate in being rapidly filled so the winds will begin easing each passing day. But it looks like another upper high pressure builds over us Friday so expect things to heat up a bit.

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